1. Field of the Invention
The present invention relates to an X-ray CT (Computed Tomography) apparatus. More particularly, in a multi-slice X-ray CT apparatus that uses a plurality of rows of detectors for detecting an X-ray image formed by helical scanning of the surrounding of a subject to be examined, this invention relates to a technique for achieving a data interpolation and an image reconstruction based on data by helical scanning of the subject by tilting a patient couch or a gantry.
2. Description of the Background Art
There have so far been proposed X-ray CT apparatuses using a helical scanning system. The X-ray CT apparatus based on the helical scanning system collects tomographic image data of a subject to be examined 12 by moving a patient couch to a body axial direction (hereinafter to be referred to as a Z-axial direction) of the subject 12 in synchronism with a continuous rotation of both an X-ray focus 13 and a detector 11, as shown in FIG. 1. In other words, in the helical scanning system, X-ray CT apparatus moves the patient couch to a body axial direction of the subject 12 through a center of the rotation of the X-ray focus 13 and the detector 11 while rotating these units. Accordingly, it can be understood that the X-ray focus 13 and the detector 11 take a spiral locus around the subject 12. On the other hand, FIG. 2 is a view for explaining a conventional scanning system for collecting data by moving the patient couch for each rotation of the X-ray focus and the detector. As compared with the conventional scanning system, the helical scanning system achieves a scanning at a higher speed over a wider range.
The X-ray CT apparatus based on the helical scanning system is further broadly divided into two kinds, that is, a single-slice CT apparatus and a multi-slice CT apparatus, based on a structure of the detector.
The first single-slice CT apparatus has an X-ray beam generation source for irradiating fan-shaped X-ray beams (hereinafter to be referred to as fan beams), and a detector having M channels (for example 1,000 channels) arrayed in a fan shape or in a linear shape in one row. This single-slice CT apparatus has the X-ray beam generation source and the detector rotated around the subject, and collects M data (for example of 1,000 data) in one rotation. Data collection in one time is called one view.
The second multi-slice CT apparatus has an X-ray beam generation source for irradiating conical X-ray beams (hereinafter to be referred to as cone beams), and a two-dimensional detector having detectors arrayed in a Z-axis (body axis) direction in a plurality of rows, each detector having an arcuate array of M-channel detectors (M channels times N rows). FIGS. 3A, 3B and 3C show detectors, each detector having two rows, four rows and eight rows, respectively. The multi-slice CT apparatus rotates the X-ray beam generation source (X-ray focus) 13 and the detector 32 around the subject, and collects M times N data in one rotation. Accordingly, as compared with the first single-slice CT apparatus, it is possible to scan over a wide range in higher precision and at a higher speed.
In the coordinates of scanning in FIG. 4, the Z-axis (body axial direction) coincides with a slice direction in which the slicing proceeds.
FIG. 5 is a view for showing the scanning of the multi-slice CT apparatus as observed from a Z-axial direction. In the drawing, a reference numeral 51 within a circle represents an effective field of view diameter FOV (Field of View). A reference numeral 52 placed between the X-ray focus 13 and the center of the FOV represents a distance between the X-ray focus and the rotation center, FCD (Focus Rotation Center Distance). A reference numeral 53 represents a fan angle. FIG. 6 is a view of a four-row multi-slice CT as observed from a direction perpendicular to the Z-axis including the Z-axis. A beam thickness 61 in the Z-axial direction, when X-rays incident from the X-ray focus 13 to the detector element 32 has passed through the rotation center (that is, FCD 52), is expressed as a reference slice thickness T. In FIG. 6, a central slice exists between the second-row detector and the third-row detector. A couch travel distance in one rotation is called a helical pitch. A helical pitch P (as represented by 62) in the multi-slice CT becomes a product of the number of detector rows N times the reference slice thickness T.
Next, an outline of an image reconstruction processing in the helical scanning system will be explained. In the following explanation, the subject 12 having only an arrow signal around the rotation is considered as shown in FIG. 7.
(1) Projection Data Collection Processing
First, as shown in FIG. 8A, projection data collected by the detector at each view of the helical scanning is collected for all angles. The projection data is corrected by taking into consideration the sensitivity of the detector, the X-ray intensity and various other physical factors. The data after the correction is called raw data.
(2) Helical Interpolation Processing
Second, in the case of the helical scanning, interpolation is conducted based on the raw data in a Z-axial direction, to generate interpolated data on a desired slice surface. This interpolation is called a helical interpolation. This processing is carried out, as only data of one view is collected on the targeted slice surface according to the helical scanning. The interpolation processing will be explained in detail later.
(3) Convolution Processing
Third, as shown in FIG. 8B, the interpolated data for the respective angles are subjected to convolution calculation of a reconstructing function (a filter function). FIGS. 10A, 10B, 10C and 10D show examples of shapes of filters. These filter shapes are selected according to the characteristics of the image data to be obtained. The convoluted data after the calculation exhibits a shape with a decay on the surrounding for an actually existing signal.
(4) Back Projection and Fan Beam Reconstruction Processing
Fourth, the convoluted data is added to all the pixels which are arrayed along the path of an X-ray beam at the time of data collection. FIG. 9 shows a result of the back projection calculation at a certain angle. When this back projection is repeated for the convoluted data at necessary angles according to the beam shape, only the original signal remains, and desired image data is fan-beam reconstructed.
An interpolation method in the case of carrying out a helical scanning in the multi-slice CT apparatus will be explained next. As such an interpolation method, there exists an adjacent interpolation method as disclosed in Japanese Laid-open Publication Hei 4-224736. FIG. 15 shows a conceptional diagram of the adjacent interpolation method for the case where the helical pitch is 4 in the four-row multi-slice CT. According to this adjacent interpolation method, real data or opposite data corresponding to the real data at two adjacent points in a Z-axial direction (slice direction) at a target slicing position, are used for linear interpolation with an inverse ratio of a distance between the target slicing location 151 and the sampling position. In this case, the real data is equivalent to the raw data. This adjacent interpolation method is a method employed by extensively applying a 360-degree interpolation method used for the single-slice CT apparatus. As shown in FIG. 11, according to the 360-degree interpolation method, real data 152 and 153 of two views which are in same phase with each other at the nearest positions and sandwiching a target slice plane 151, are used for linear interpolation with an inverse ratio of a distance between the slice plane and the sampling position. This processing is repeated for all the necessary phases.
Further, in Japanese Laid-open Publication Hei 9-234195, there is disclosed a filter interpolation method for performing an addition of weighted multi-point data. FIG. 16 shows a conceptional view of the filter interpolation method. According to this filter interpolation method, real data group and/or opposite data group opposite to the real data group are filtered (added with weight) in the Z-axial direction (slice direction), thereby obtaining data of a target slicing position 151.
Further, an opposite beam interpolation method which is used in the single-slice CT apparatus can also be used. According to this opposite beam interpolation method, opposite data are formed which are virtual data based on opposite beams shown by broken lines in FIG. 12 extracted from each focal position. This is a two-point interpolation method for linearly interpolating between the opposite data and the real data as shown in FIG. 13. FIG. 14 is a view for explaining a sampling position of an opposite beam. In the above-described Japanese Laid-open Publication Hei 9-234195, a new opposite beam interpolation method which is an extended application of this opposite beam interpolation method is disclosed. FIG. 17 and FIG. 18 show conceptional views of the new opposite beam interpolation method. According to this new opposite beam interpolation method, interpolated data of a target slice is obtained by interpolating between the two nearest beams by sandwiching a slice surface from all the beams regardless of the opposite data or the real data. A shaded area in FIG. 15 shows one example of a data sampling range according to the adjacent interpolation method using the real data in the multi-slice CT. Shaded areas in FIG. 17 and FIG. 18 show one example of a data sampling range for the interpolation using the real data and the opposite data (new opposite beam interpolation method).
In the case of carrying out the helical scanning in the multi-slice CT apparatus, interpolated data is generated and image reconstruction is performed by using the above-described various helical scanning methods.
However, the conventional multi-slice CT apparatus has the following problems.
In the clinical operation, image reconstruction is usually performed by collecting data based on not only a scanning of a perpendicular slice surface but also based on a scanning of a tilted slice surface, not perpendicular to a body axial direction (couch moving direction), by tilting the gantry. This scanning is called a tilt scanning. A slice plane in the case of the tilt scanning is called a tilt plane.
Coordinate system of a tilt scanning will be defined by using FIG. 19. When a tilt angle of the gantry is set as a tilt angle xcex1, a Zxe2x80x2 axis is defined with a tilt of the tilt angle xcex1 with respect to the body axis (Z axis). In FIG. 19, the Zxe2x80x2 axis is a travel direction of the slice, and this is defined as perpendicular to a gantry rotation plane 191 (that is, the tilt plane) including a tubular bulb and a detector. The X-axis is a straight line formed by crossing two slice planes before and after the tilt. The coordinate system is structured by Yxe2x80x2-axis perpendicular to the X-axis and the Zxe2x80x2-axis respectively, and Y-axis perpendicular to the X-axis and the Z-axis respectively. In FIG. 19, the couch moves in the Z-axial direction. On the other hand, the gantry travels in a Zxe2x80x2-axial direction by scanning the adjacent slices as shown by dotted lines. The coordinate system of FIG. 19 can be applied to arbitrarily desired tilt direction and tilt angle. As can be understood from the coordinate system in the tilt scanning shown in FIG. 19, the Z-axial direction does not coincide with the Zxe2x80x2-axial direction (slice direction), and a predetermined tilt angle xcex1 is formed.
However, the above-described various conventional helical interpolation methods can be applied to only the case where the body axial direction in which the couch moves and the slice plane forms a perpendicular angle. Accordingly, there is a problem that these interpolation methods cannot be applied when a helical scanning is carried out by tilting the gantry in the multi-slice CT.
The reasons are as follows. When a helical scanning is carried out by tilting the gantry by only the angle xcex1 in the multi-slice CT apparatus, the rotation center of each detector row of the couch is deviated to up and down directions, that is, in a Yxe2x80x2-axial direction or a Y-axial direction. This deviation will be explained based on FIG. 20. Fan beams 201 shown by thick lines in FIG. 20 are an X-ray focus and X-ray paths in an n-th rotation with respect to a detector in the first row. On the other hand, fan beams 202 shown by thin lines in FIG. 20 are an X-ray focus and X-ray paths in an n-th rotation with respect to a detector in the second row. As can be easily understood from FIG. 20, the X-ray paths of the detector rows for the same channel are deviated (deviated to a moving direction of the couch) when observed from a Z-axial direction. Therefore, according to the data collected based on the X-ray beams irradiated from such different focal positions, the X-ray paths extending in a fan shape are deviated.
In this case, a deviation to a Yxe2x80x2-axial direction (Shift Yxe2x80x2 (n)) and a deviation to a Y-axial direction (Shift Y(n)) from the central slice (midplane) shown in FIG. 6 are given by the following Expression 1 and Expression 2, respectively.
Shift Yxe2x80x2(slice, n, a)=Zt(slice, n)xc3x97tan(xcex1)=slicexc3x97(Ncxe2x88x92n)xc3x97tan(xcex1)xe2x80x83xe2x80x83(Expression 1)
Shift Y(slice, n, a)=Zt(slice, n)xc3x97sin(xcex1)=slicexc3x97(Ncxe2x88x92n)xc3x97sin(xcex1)xe2x80x83xe2x80x83(Expression 2)
where N represents a number of rows of collection, Slice represents a thickness of slice in each row, Nc represents a central slice of equal Zxe2x80x2-axis coordinates to X-ray focus, and Zt (n) represents a distance from the central slice to each slice on the Zxe2x80x2-axis coordinate.
In the above-described helical interpolation, it is necessary to interpolate between the data each having the same constant distance from a certain pixel to a focus, in order to obtain reconstructed image data with practical picture quality having eliminated any blurs. For this purpose, the two data between which the interpolation is carried out need to be the data on the same path coming from the same focus, when observed from the Zxe2x80x2-axial direction in which the interpolation is carried out. In other words, it is necessary to use the data having no deviation in the X-Yxe2x80x2 direction and being deviated in only the Zxe2x80x2-axial direction, as the base data for interpolation.
However, when the above-described tilting of the gantry is carried out, the collected data of respective rows serving as two-point or multi-point real data groups (or real data and opposite data) for generating the interpolated data, have their X-ray focus and X-ray paths deviated in a X-Yxe2x80x2 plane direction. In other words, the data of an identical view angle and an identical ray angle (that is, channel angle) as those of the other data between which the interpolation is to be carried out, is deviated in the X-Yxe2x80x2 plane direction. Therefore, there is no data between which the interpolation can be performed.
Also, in the case of the above-described single-slice CT apparatus, the gantry rotation plane and the couch moving direction including the tubular bulb and the detector are not perpendicular to each other because of the tilting of the gantry. However, as only one detector row exists in the single-slice CT apparatus, there occurs no deviation in the X-ray paths. Accordingly, in the case of the single-slice CT, it is possible to carry out the image reconstruction based on the usual fan-beam direct back projection method or the like, by helically interpolating between the data of an identical view angle and an identical channel angle (that is, a ray angle) while disregarding a tilt of the tilt angle xcex1.
As explained above, when a helical scanning is carried out in the multi-slice CT apparatus, it has not been possible to perform an image reconstruction based on a method of helical interpolation and image reconstruction in the multi-slice CT apparatus as shown in FIG. 15 to FIG. 18, such as, for example, the method as described above for performing a helical interpolation by taking out data for one rotation, and performing a filtered back projection based on the fan beam direct back projection method. Therefore, it has not been possible to implement a helical scanning by tilting the gantry in the multi-slice CT apparatus.
The present invention has been developed in order to solve the above-described problem that it is not possible to perform a helical interpolation when it is desired to carry out a helical scanning by tilting a gantry in a multi-slice CT apparatus, as there occurs a deviation in the X-ray path in each row of a detector.
It is an object of the present invention to provide an X-ray CT apparatus capable of realizing an image reconstruction based on a helical scanning by tilting a gantry in a multi-slice CT apparatus.
An aspect of the present invention is in that data collected based on fan beams are converted into data of parallel beams (this processing will hereinafter be referred to as a fan beam data-parallel beam data conversion), thereby eliminating a focus, and that the data after converted into parallel beam data are subjected to a positional correction based on a tilt amount.
According to one aspect of the present invention, as shown in FIG. 21, there is provided an X-ray computed tomography apparatus, comprising: a couch on which a subject to be examined is to be placed; a gantry, including an X-ray source for generating X-rays, and a detector having detector elements laid out in a plurality of rows in a slice direction for detecting X-ray beams transmitted through the subject; a data collector for collecting helical data by the detector, by rotating the X-ray source while moving at least one of the gantry and the couch along a body axial direction of the subject in a state that at least one of the gantry and the couch is tilted; and a data processor for reconstructing an image by interpolating between the helical data collected by the data collector.
The data processor may interpolate between the helical data based on a tilt angle of the couch or the gantry. The data processor may interpolate between the helical data in a slice direction or in a body axial direction.
According to the above-described structure, in a multi-slice CT apparatus, it becomes possible to collect projection data by carrying out a tilt helical scanning, and reconstruct an image by a helical interpolation based on the collected projection data and tilt data. In other words, in the multi-slice CT apparatus, it becomes possible to carry out a helical scanning capable of collecting data over a wide range at a high speed by tilting the gantry or the couch.
Further, the data processor may include: a data converter for converting helical data collected by the data collector into parallel beam data; and a shift data calculator for calculating shift data that corrects a deviation of X-ray paths generated by the tilting of the couch or the gantry.
When the fan beam data-parallel beam data conversion is used, the data collected by the helical scanning (herein after referred to as helical data) are converted into the parallel beam data so that a focus of the X-ray paths is eliminated. Thus, it is possible to carry out a helical interpolation by easily correcting positions of the collected data of each row of the detector on the X-Yxe2x80x2 plane.
The data converter converts fan beam data of each view angle into the parallel beam data by selecting each X-ray path data that is parallel with the reference path. Thus, it becomes possible to easily generate the parallel beam data at a high speed from the collected data, and to select data for interpolation.
With the above-described structure, it becomes possible to easily calculate a positional correction amount of the data collected for each row of the detector on the X-Yxe2x80x2 plane and to obtain reconstructed image data at a high speed and in high picture quality.
Further, with the above-described structure, it becomes possible to apply various helical interpolation methods by suitably selecting data, without being conscious that the data is helical scan data obtained by tilting the gantry.
As a helical interpolation method, it is possible to use so-called a filter interpolation method for obtaining interpolated data by adding weighted multi-point sampled data.
Further, with the above-described structure, it becomes possible to obtain reconstructed image data of a small effective slice thickness and in high picture quality, by decreasing deterioration in the picture quality due to a change-over of beams that are used for the interpolation.
The correction based on the shift data may be carried out during a generation of parallel beams, or during a helical interpolation or during an image reconstruction.
According to the above-described structure, it is possible to carry out a helical interpolation by easily correcting on the X-Yxe2x80x2 plane the positions of the parallel beam converted data collected for each row of the detector, and to obtain reconstructed image data at a high speed.
It is also possible to improve the parallel processing of the image reconstruction processing by carrying out a helical interpolation based on a conversion of data collected by helical scanning into parallel beam data and thus eliminating a focus of X-ray paths, and by generating reconstructed image data based on a positional correction of interpolated data for each view angle during a back projection processing.
The conversion of the helical data into the parallel beam data can be carried out by the data converter, and the data converter selects each X-ray path data that is parallel with the reference path, for each fan beam data at each view angle.
The shift data is obtained based on a tilt angle formed by the rotation plane of the gantry and a slice direction or a body axial direction perpendicular to the rotation plane. Further, the shift data is obtained based on at least one of the tilt angle, the slice thickness, the view angle, and the number of rows of the detector. Further, the shift data is obtained based on a relative distance between the central slice and each detector row.
The data processor may generate interpolated data by adding weighted multi-point sampled data.
The data processor may carry out convolution and back projection to each of data of identical view angle, and reconstruct an image by superimposing the projected data.
The data processor may further include a data corrector for correcting a deviation of slicing positions of the parallel beam data.
According to the above-described structure, it is possible to obtain a reconstructed image of higher picture quality by correcting the deviation of slicing positions of each parallel beam data.
Other features and advantages of the present invention will become apparent from the following description taken in conjunction with the accompanying drawings.